The invention relates to a method for data transmission in a wireless communication system in which a subscriber data signal is emitted on the transmit side by way of at least two antenna devices.
With regard to wireless communication systems, in order to improve the quality of a data transmission so-called “diversity methods”, referred to for example as “space diversity methods” or as “polarization diversity methods”, are used.
With regard to a space diversity method used on the transmit side, a carrier frequency subscriber data signal to be emitted is delivered as a user data signal to at least two antenna devices which exhibit a difference of several wavelengths between one another and which have the same polarization.
With regard to a polarization diversity method used on the transmit side, the subscriber data signal to be emitted is similarly delivered to at least two antenna devices which however exhibit different polarizations. Typically, two antenna devices are located in a common antenna housing.
Diversity methods can be used both on the transmit side and also on the receive side and serve to enhance the transmission quality by enhancing an observed receive situation.
With regard to wireless communication systems, such as for example in the case of the GSM mobile radio system or in the case of the GERAN mobile radio system, a transmit-side subscriber data signal for example is divided into two partial signals which are then delivered by way of two “carrier units” to two spatially separated antenna devices having the same polarization for emission. Since as a result of their design the carrier units exhibit tolerances in the respective signal paths of the partial signals, the two partial signals are subject to different signal propagation delays in respect of emission. In addition, specific propagation paths having different signal propagation delays and signal attenuations are produced for each individual partial signal in the radio field as a result of multipath propagation.
On the receive side, a superimposition of the individual partial signals takes place with respect to the subscriber signal, whereby a so-called “diversity gain” is achieved in systems engineering terms. On the other hand, a radio cell enlargement or a range extension can be achieved between sender and receiver by way of the diversity gain.
With regard to the receiver on the other hand, the different propagation paths should be taken into consideration in an appropriate manner, which implies an increased complexity on the part of the receiver.
With regard to mobile radio systems, such as for example in the case of the GSM mobile radio system, a positional determination (location service) is carried out for the subscriber during a data transmission between a mobile subscriber and a base station, using the so-called “Timing Advance Mechanism, TA” for example. In this situation, signal propagation delays for a reference signal are determined during the data transmission between subscriber and base station and these are used to ascertain the position of the subscriber.
Inaccuracies in the positional determination can be attributed directly to inaccuracies occurring whilst determining the signal propagation delay of the reference signal.
With regard to the GSM mobile radio system, by using the TA mechanism it is possible to realize positional determinations having an accuracy of about 200 meters, whereby in addition to the TA mechanism further standardized methods such as Assisted GPS (A-GPS), Enhanced Observed Time Difference (E-OTD) and Cell ID Timing Advance (CITA) are known for positional determination.
A positional determination can be carried out with a required level of accuracy in respect of a diversity method executed on the transmit side only with a high resource requirement, or cannot be carried out at all, as a result of the multipath propagation and the different signal propagation delays in the respective carrier units.
Corresponding problems occur for runtime dependent or runtime critical system parameters or system properties in respect of the data transmission, for example in the case of a “synchronized handover” or a “pseudo-synchronized handover”.
A so-called hybrid transmit diversity method for transmission of adjacent, successive time slots is known from WO 02/11315 A2. In this situation, information is transmitted from a base station to a mobile terminal X during a first time slot with the aid of a so-called “Delay Diversity” method, while information is transmitted to a further mobile terminal Y with the aid of a so-called “Space Time Diversity” method.
A base station having a plurality of transmit antennas is known from US 2002/0022502 A1. In this situation, a unidirectional channel is transmitted either by a first antenna or by a second antenna. The switchover between the two antennas takes place with the aid of a predetermined random selection.
Different transmit diversity methods for a CDMA-TDD wireless communication system are known from “Transmit Diversity Applied on the CDMA/TDD Cellular Systems”, Hiramatsu et al, VTC 2000-Spring. 2000 IEEE 51st Vehicular Technology Conference Proceedings, Tokyo, Japan, May 15–18, 2000, Vol. 2 OF3, pp. 1170-1174, XP000968054. In this situation, for example the physical synchronization channel PSCH is transmitted by a “Time Switched Transmit Diversity” method in which the PSCH is emitted in alternate succession by way of two antennas. The Primary Common Control Channel P-CCPCH is transmitted by a “Block Space Time Transmit Diversity” method in which the P-CCPCH is fed simultaneously to two antenna branches, whereby a separate coding occurs in each antenna branch and the signals differing in their coding are emitted simultaneously by way of two antennas. The Dedicated Physical Channel DPCH is transmitted by a “Selective Transmit Diversity and Transmit Adaptive Antennas” method in which the DPCH is emitted simultaneously with differing weighting by way of two antennas.
A positional determination method for a subscriber device in a wireless communication system is known from DE 100 31 178 A1. With regard to this method, a distinction is made between time critical data on the one hand and time non-critical data on the other hand, whereby the time critical data is transmitted during a time critical window and the time non-critical data is sent during a time non-critical window. Measurement signals required for positional determination are sent during the time non-critical windows in order not to adversely affect the transmission of time critical information. The time critical information is divided into time slots and transmitted in periodically recurring frames. Reference signal transmission is performed for example only in the case of every n-th frame. Transmission resources are saved as a result.
While it is true that in the case of n=I where a reference signal transmission to an observed subscriber occurs in each frame a positional determination for example would be extremely precise, a reliable wireless delivery to the subscriber would however become uncertain as a result of the constant loss of the diversity gain.